1. Field of the Invention
This invention relates to image forming apparatuses using a speed reduction device with elastic bodies, and for example, relates to an image forming apparatus using a speed reduction device with elastic bodies that reduce the rotational speed of a motor, which rotates a photoreceptor drum, transfer belt and other components in a copying machine and so forth, by means of frictional contact of the elastic bodies.
2. Description of Background Art
Image forming apparatuses such as copying machines for black and white printing require only a single-color drum to be rotated, thus variations in the rotational speed of the drum are acceptable to some degree. However, color printing requires creating four different color images by rotational drums and superimposing the images. Because of this, the variations in the rotational speed of the drums to create each color image cause color registration errors and color nonuniformity. In order to prevent such a color registration error and nonuniformity, there is a need for a rotation drive device rotating photoreceptor drums, transfer belt and so forth at a relatively low speed and with high accuracy.
FIG. 5 is a cross-sectional view showing such a conventional rotation drive device. This rotation drive device is disclosed in Japanese unexamined patent publication No. 2002-115751. In FIG. 5, the rotation drive device includes a motor 1, a speed reduction device 2 and a speed detection mechanism 3. The speed reduction device 2 employs a traction system (friction transmission system), which is allegedly advantageous in reducing the rotational variations.
An end of a rotary shaft 14 in the motor 1 acts as a sun roller 20 and makes contact with a plurality of planetary rollers 22. Each planetary roller 22 is cantilevered by a rod 28 from a carrier 23. The planetary rollers 22 are in contact with an inner surface of an internal ring 21 via elastic bodies 31 such as rubber. The rotation of the rotary shaft 14 driven by the motor 1 produces a torque which is reduced by the sun roller 20, planetary rollers 22 and internal ring 21 at their reduction ratios depending on the individual external and internal diameters, and the reduced torque is then output through the carrier 23 and an output shaft 24.
The output from the speed detection mechanism 3 is input to a controller 4. Based on a control signal from the controller 4, a drive device 5 controls the rotational speed of the motor 1. Since the planetary rollers 22 used in the speed reduction device 2 make contact with the internal ring 21 via the elastic bodies 31 which may cause generation of a delay element in a feedback control loop, the controller 4 should control the drive device 5 in a manner that the drive device 5 will not be uncontrollable due to the delay element.
For this purpose, the controller 4 adopts a feedback control as disclosed in Japanese unexamined patent publication No. 2002-171779 (hereinafter referred to as “patent document”). Specifically, a rotational speed output from the speed reduction device 2 is detected to obtain a difference value from a target speed. The controller 4 feeds a speed command signal based on the difference value to the motor 1 to directly control the motor's rotational speed, therefore suppressing delay factors.
The speed reduction device 2 utilizing the frictional force of the elastic bodies 31 has another problem. In order to reduce speed by means of the frictional force, the elastic bodies 31 and planetary rollers 22 must be applied with pressure, thereby causing deformation of a pressurized part of the elastic bodies 31. During operation of the speed reduction device 2, that is to say when pressure continues to be applied successively along the circumference of the planetary roller 22, the elastic bodies 31 by deforming continuously to their original shape without partial deformation. In this case, the problem can be avoided through the control to reduce the delay factors caused by the deformation, as disclosed in the patent document.
However, once the speed reduction device 2 is stopped, only some parts of the elastic bodies 31 stay under pressure and then become deformed. The elastic bodies 31 that are applied with the pressure for a shorter time take a shorter time to be restored, while the elastic bodies 31 that are applied with the pressure for a longer time take a longer time to be restored. Thus, in the case where the speed reduction device 2 is stopped and then rotated again, during the interval until the deformed parts return to their original shape, the rotational speed fluctuates every time the deformed parts transmit drive. Depending on the structure, the rotational speed fluctuates every time the elements of the speed reduction device 2 come to a halt position.
The time of the rotation variations caused by the deformed part of the elastic bodies 31 caused by the halt is usually much shorter than a response frequency of the feedback control. Because of this, if the feedback control system disclosed in the patent document is used to detect the deformed parts of the elastic bodies 31 produced during the halt in order to exercise the feedback control, an excessive control will occur when the speed reduction device 2 is continuously driven under the feedback control. Therefore, the feedback control utilizing the deformed part of the elastic bodies 31 caused by the halt is not effective.
The rotation variations of the speed reduction device 2, which is used to drive the photoreceptor drum or transfer belt in the image forming apparatus, generated during image formation provide a fatal effect on the formed image. In a color copying machine, for example, a color registration error may occur simultaneously with the speed variations.
When the speed reduction device 2 with the elastic bodies 31 used in the rotation drive device is activated again after a halt, the rotation variations occur due to the partial deformation of the elastic bodies 31 caused by the halt, which is an unavoidable problem that cannot be solved by the usual feedback control. However, the rotational variations generated during the image formation cause image degradation, and therefore must be avoided.
The elastic bodies 31 that are deformed within their deformable limit can return to their original shape over time after eliminating factors responsible for the deformation. This means that the rotational variations will disappear after a predetermined lapse of time required for the speed reduction device 2 to restore to its original shape. In the case where the speed reduction device 2 stops for a long time and the elastic bodies 31 are possibly deformed, the rotational variations during the image formation can be avoided by letting the image formation wait until the deformation of the elastic bodies 31 are restored after the resumption of rotation.
The recovery time of the deformed part would vary depending on the type of the elastic bodies 31 in use, pressure applied to the elastic bodies 31 and other conditions such as temperature and humidity. Therefore, variations of the individual speed reduction device 2, usage environment and so forth must be considered all together in order to determine the actual length of standby time. However, the rotational variations may not occur depending on the individual difference of the speed reduction device 2 and the usage environment, because the elastic bodies 31 may have been restored before the above-determined standby time elapses. In this case, the processing capacity for image formation will be decreased by the wasted standby time.